Well first off you need the Sims 3 base game in order to play the expansion packs. Supernatural and Pets (along with the others) are expansion packs, and you need the base game installed in order to play them.

To answer your question, I think Late Night and Seasons are the better ones. Late Night adds bars, vampires, clubs, your sims can join a band, etc,. Seasons adds weather to the game, along with different activities for your sims to do during each season.